I'm not sure if this has been brought up or not, and it may not work for everyone, we all have our differences in body type.

I found that if I follow the instructions packet to the letter. The part about laying on one side and cocking one knee up higher than the other. I find that that position forces the abutement tab to slide off center. If I switch legs, it slides the opposite direction. Keeping my knees in alignment, and hips aligned in the same direction as my shoulders, is what puts it dead center on me.